PDPs are self-luminescent type of display devices advantageous in the display brightness, and have been attracting attention as a display device for replacing CRTs, owing to their potentiality of large screen size and their high-speed displaying capability. Particularly, surface-discharge type PDPs suitable for color displays employing fluorescent materials have been rapidly increasing their application areas in the field of television picture including the high definition television.
FIG. 1 shows an exploded perspective view of a general surface-discharge type PDP, in which is shown a basic structure of a part which corresponds to a single picture element EG. The PDP1 shown in FIG. 1 is of a three-electrode structure called a reflection type in the classification of fluorescent materials arrangement, employing a pair of glass substrates 11 and 21; pairs of display electrodes X & Y provided thereon extending in the lateral direction in parallel adjacently to each other; a dielectric layer 17 for an AC drive which utilizes wall charges for a discharge; a protection film 18 formed of magnesium oxide (MgO); address electrodes A orthogonal to display electrodes X & Y; separator walls 29 which are like lines in parallel to address electrodes A when looked down; and fluorescent material layers 28 to display primary colors, red (R), green (G) and blue (B), respectively.
Separator walls 29 divide an internal discharge space 20 into unit lighting-areas EU in the extending direction of display electrodes X & Y, and define the gap dimension. Fluorescent material layers 28 are provided between each separator wall on a glass substrate 21 opposite from display electrodes X & Y in order to avoid ion bombardment of the surface discharge, and emits a light by being excited by an ultra violet light generated in the surface discharge. A light emitted at the surface plane (a surface which faces the discharge space) penetrates dielectric layer 17 and glass substrate 11, etc. so as to radiate outwardly from display surface 4.
Display electrodes X & Y being arranged on a display surface H which opposes fluorescent material layers 28 are formed of a wide and transparent electrode 41 and a narrow metal film (a bus electrode) 42 for supplementing the electrical conductivity, in order to perform the surface discharge in a wide area and to minimize the light shielding. Transparent electrode 41 is formed of metal oxide, such as ITO (indium oxide) and NESA (tin oxide). A typical example of this kind of AC type surface discharge PDP was disclosed in European Patent Application No. 0 554 172A1.
For thus constituted PDP, its smoother surface plane is desirable in securing a uniform discharge characteristics and the transparency.
Accordingly, dielectric layer 17 is generally formed of a single glass layer such that a low-melting temperature lead-glass (containing about 75% of PbO) having a melting temperature of, for example about 470.degree. C., is fired at a temperature 600.degree. C. adequately higher than its softening temperature. The high temperature firing at the temperature adequately higher than its softening temperature allows the glass material to flow during the firing so as to accomplish a glass layer having a flat surface.
In driving PDPs, the equality of the electric potential status between display electrodes X & Y is deteriorated when the pulse widths of the driving pulses, applied to the display electrodes X & Y of each pair, are subtly imbalanced or when such a sequence is constantly employed that the number of the pulses applied to one of display electrodes is more than those to the other one. That is, a DC voltage of, for example, about 200 V of the same polarity comes to be applied thereto for a considerable period. On the other hand, the gap between display electrodes X & Y is as small as 100 .mu.m. And, the dielectric layer 17 to insulate them contains PbO as described above. It is estimated that the surface of dielectric layer 17 upon whose surface a discharge takes place becomes a considerably high temperature. Really, the glass surface reaches 70.degree. C. Moreover, indium and tin included in the transparent electrodes are chemically unstable, and also the copper of the metal electrodes are materials which easily penetrates into dielectric layer 17 so as to cause electro-migration. Combination of the electrode material, insulating material, the applied high electric field and the high temperature satisfies the condition to accelerate the electro-migration.
A long term operation, under such a condition of the prior art structure causes the electro-migration of display electrodes X & Y to progress such that a tree-like spike is grown in dielectric layer 17, from transparent electrode film 41 of one of the display electrodes to transparent electrode film 41 of the other one of the display electrodes. Therefore, there was a problem in that the insulation resistance was locally decreased, whereby a unit lighting area EU that should not light erroneously lights. It is impossible to completely remove the imbalance of the applied voltages which is the cause of the electro-migration.